Desirable fluorescent lamp characteristics are high brightness and high color rendering at an economical cost. To achieve this goal, such lamps as the Sylvania.TM. Octron and Designer series of lamps are constructed with two layers of phosphor coatings. The first or the base coat is an inexpensive halophosphate phosphor of the desired lamp color temperature. The second or skin coat is comprised of three expensive rare earth activated phosphors, emitting in the red, green and blue spectral regions, blended to effect a composite white emission of desired color temperature. In this configuration the expensive tri-phosphor blend absorbs the ultra-violet excitation energy of the Hg plasma in excess proportion to the weight of the phosphor in the lamps. The halophosphate base coat absorbs the excitation energy that eludes the skin coat, while diluting the high CRI and brightness capability of the tri-phosphor blend. Even though these phosphor blends achieve desirable economic and performance characteristics, further improvements are desirable.
U.S. Pat. No. 4,623,816 to Hoffman et al relates to a fluorescent lamp utilizing a dual phosphor layer coating having a conventional calcium haloapatite phosphor and a top phosphor layer comprising a tri-phosphor blend including a lanthanum cerium orthophosphate phosphor activated with terbium ion as the green color component along with an europium-activated yttrium oxide phosphor as the red color component.
U.S. Pat. No. 4,296,353 to Walter relates to a fluorescent lamp having a coating on the inner surface of the glass envelope comprising a blend of four narrow band emitting phosphors. The spectral power distribution curves for the phosphor blends consist of four narrow bands centered at about 450-480 nm, 510-540 nm, 570-590 nm, and 600-630 nm. The particular phosphors utilized were divalent europium activated barium magnesium aluminate, manganese activated zinc orthosilicate, trivalent dysposium activated yttrium vanadate, and europium activated yttrium oxysulfide. Phosphor blends include a Cool White lamp, Warm White lamp, and Daylight lamp.
A skin coat or tri-phosphor blend that has been used is a red Y.sub.2 O.sub.3 :Eu.sup.+3 (Sylvania Type 2342), a green CeMgAl.sub.11 O.sub.19 :Tb.sup.+3 (Sylvania Type 2297), and a blue BaMg.sub.2 Al.sub.16 O.sub.27 :Eu.sup.+2. Fluorescent lamps utilizing the above skin coat have achieved high color rendering and high brightness while demonstrating excellent durability in the harsh environment of the fluorescent lamp. However, additional and further improvements are desirable. Especially desirable is the production of a blend which produces efficient white color emission and improved color rendition at an even more economical cost.
The luminous efficacy, color rendering index and other lamp output characteristics may be varied depending upon the particular composition of the lamp phosphors utilized. Certain terms as used in this specification have meanings which are generally accepted in the lighting industry. These terms are described in the IES LIGHTING HANDBOOK, Reference Volume, 1984, Illuminating Engineering Society of North America. The color rendering index of light source (CRI) is a measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of comparable color temperature. The CRI rating consists of a General Index, R.sub.a, based on a set of eight test-color samples that have been found adequate to cover the color gamut. The color appearance of a lamp is described by its chromaticity coordinates which can be calculated from the spectral power distribution according to standard methods. See CIE, Method of measuring and specifying color rendering properties of light sources (2nd ed.), Publ. CIE No. 13.2 (TC-3,2), Bureau Central de la CIE, Paris, 1974. The CIE standard chromaticity diagram includes the color points of black body radiators at various temperatures. The locus of blackbody chromaticities on the x,y-diagram is known as the Planckian locus. Any emitting source represented by a point on this locus may be specified by a color temperature. A point near but not on this Planckian locus has a correlated color temperature (CCT) because lines can be drawn from such points to intersect the Planckian locus at this color temperature such that all points look to the average human eye as having nearly the same color. Luminous efficacy of a source of light is the quotient of the total luminous flux emitted by the total lamp power input as expressed in lumens per watt (LPW or lm/W).
The present invention addresses the problem of how to economically elevate color rendering while retaining high light flux.